1. Field of the Invention
The present invention relates generally to an ElectroPhoretic Display (EPD), and more particularly, to a method and an apparatus for driving an EPD in accordance with an ambient temperature.
2. Description of the Related Art
The concept of electronic paper incorporates a new display device having advantages of existing display devices and printed paper. Electronic paper is reflective display, which has the most superior viewing characteristics among display media, such as, high resolution, wide viewing angle, and bright white background, like the existing paper and ink. Electronic paper can be implemented on any substrate, such as plastic, metal, paper, and the like. Electronic paper maintains an image even after the power supply is interrupted via a memory function, and requires no backlight power. Thus, the life span of a battery of a mobile communication terminal can be lengthened, and the manufacturing cost and the weight of the terminal can be reduced. Additionally, since electronic paper can be implemented in a wide area in the same manner as existing paper, it can be applied to a larger-scale display.
Electronic paper can be implemented using an EPD. The EPD displays data in white or black in accordance with an applied voltage, and is constructed through the application of electrophoresis and microcapsules. A general cell structure of such an EPD is illustrated in FIG. 1. FIG. 1 is a sectional view illustrating an operation principle of the EPD. The EPD is constructed by manufacturing a transparent microcapsule having black particles 40 and white particles 30 included in a colored fluid. The microcapsule is combined with a binder 50, and then the microcapsule combined with the binder is positioned between upper and lower transparent electrodes 20 that are in contact with an inner side of a substrate 10. If a positive voltage is applied to the electrode 20, ink corpuscles that are negatively charged move toward the surface of the EPD to display the color of the corpuscles. By contrast, if a negative voltage is applied to the electrode 20, the negatively charged ink corpuscles move downward. By this method, a text or an image can be displayed.
The EPD is dependent upon an electrostatic movement of particles floating in a transparent suspension. If a positive voltage is applied, positively charged white particles 30 electrostatically move to an electrode of an observer side, and at this time, the white particles 30 reflect light. By contrast, if a negative voltage is applied, the white particles 30 move to an electrode that is away from the observer, and the black particles 40 move to an upper part of the capsule to absorb the light, so that the observer observes the black color. Once the movement has occurred at any polarity, the particles remain in their positions even when the applied voltage is interrupted, which requires the application of a memory device having bistability. An electrophoretic capsule using a single kind of particles is constructed in a manner that a transparent high-polymer capsule has white charged particles floating in a fluid that is dyed a dark color.
The movement of the black particles 40 and the white particles 30, which constitute the EPD, is affected by the level of the voltage being applied to the particles and time for applying the voltage. As the level of the voltage becomes higher, and the time for applying the voltage becomes longer, the power of moving the particles becomes greater. A graph of FIG. 2A illustrates the movement of particles constituting the EPD in comparison to the time for applying the voltage in a 25° C. environment. Referring to FIGS. 2A and 2B, the particles abruptly move in the time of approximately 250 ms, and the amount of movement decreases after the rough movement is completed.
The mobility of the EPD particles is closely affected by an ambient temperature. This is because when the charged EPD particles move, they encounter higher resistance at a temperature lower than the ambient temperature, and encounter lower resistance at a temperature higher than the ambient temperature.
For example, when the same voltage as illustrated in FIG. 2A is applied to the particles at a temperature below −10° C., the movement of the particles is shown in FIG. 2B. The movement of the particles is completed at approximately 350 ms. Thus, the reaction time is lengthened, when compared to that of the ambient temperature shown FIG. 2A. Further, the contrast of the particles is also lowered.
The reaction times of the white particles 30 and the black particles 40 differ from each other. Accordingly, if the EPD is driven by applying a voltage of the same level for the same time regardless of the temperature, the respective particles cannot completely move in a low-temperature environment. This can result in an afterimage of data previously displayed that remains on a display screen.